1. Field of the Invention
The invention relates generally to a direct access storage device (DASD) of the type including a partial-response maximum-likelihood (PRML) channel, and more particularly to a method and apparatus for servo signal demodulation utilizing the PRML channel.
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
In direct access storage devices (DASD's), it is necessary to position the transducer heads over data tracks on the disk surfaces to properly record and retrieve data. One known method employs phase encoded signal bursts that are embedded within a particular format of information that is written on the disks, and are used by some form of phase demodulator to determine the relative position of a data head from a given data track. A typical phase demodulation system for a DASD file to compare the relationship of the servo phase bursts by measuring the relative phase difference between the phase bursts. Each of the consecutive phase bursts contain several cycles of a constant high frequency signal such that the phase difference can be integrated over the length of the burst to average out the effects of signal noise. An example of how these bursts can be formatted within each data sector in a sector servo format file is shown in FIG. 7, which shows two consecutive phase bursts written in the servo burst field. As shown in FIG. 7, the phase encoded servo pattern includes a write recovery field (W/R) and a servo identification (SID) which can identify track and sector types, for example, such as, index, non-index and guardband and to identify the start of the servo position error signal (PES) burst. The relative phase detected between the two burst fields is used to derive the positioning information.
Some implementations use more than two consecutive phase bursts in the servo burst field. However, some form of relative phase detection typically is made between the multiple bursts to derive the positioning information. The phase information is usually generated by analog circuits providing an analog voltage that represents the comparative phase difference between the bursts, which is in turn converted to a digital representation of the difference by an analog-to-digital (A/D) converter and then passed on to the digital servo processor which controls a voice coil motor (VCM) driven actuator to minimize any position error.
The traditional phase demodulation method often requires a significant amount of analog circuitry and componentry used only to derive positional information once every data sector time. The required analog detection circuitry is not available in a partial-response maximum-likelihood (PRML) data channel and must be constructed for the sole purpose of phase demodulation. This circuitry adds significant cost to a DASD file in terms of both expense and space required. Also, this circuitry is highly underutilized since it is only operational during the short servo information portion of each data sector. It is desirable to eliminate as much of this circuitry as possible by using other existing circuitry in the disk file to perform the same function.
One of the more important features to many customers of a DASD file is its cost. Therefore, it is desirable to provide alternative solutions for file functions when they can be provided at a cost savings. This is becoming more critical as DASD files continue to shrink in size so that space is available at a premium, and solutions are needed to either simplify or integrate the functions that make up a file. By combining the functions of the PRML channel more directly with the servo processor, an improved and practical cost saving solution is obtained for servo signal demodulation.